The Mortacrux is a Unique Dagger in Diablo IV Season 15. It comes from Grigoire, the Galvanic Saint (Lair Boss) per one guide; Random world drop / any Lair Boss (one guide: 'drops at any Difficulty Tier, increased drop rate per Torment tier'); 3.2.1: all Uniques drop from level 1 in any Difficulty; targetable via Horadric Cube reroll.
Unique power
When consuming a Corpse, there is a [40 - 50]% chance to also create a decaying Skeletal Simulacrum that seeks enemies but cannot attack. When it dies, it explodes for 450 Shadow damage. This effect is treated as a Macabre Skill.
Where it drops
Grigoire, the Galvanic Saint (Lair Boss) per one guide
Random world drop / any Lair Boss (one guide: 'drops at any Difficulty Tier, increased drop rate per Torment tier'); 3.2.1: all Uniques drop from level 1 in any Difficulty; targetable via Horadric Cube reroll